Catalytic converter assemblies for treating exhaust gases of automotive and diesel engines contain a catalyst support structure for holding the catalyst, used to effect the oxidation of carbon monoxide and hydrocarbons and the reduction of oxides of nitrogen, the support structure being mounted within a metal housing. The support structure is generally made of a frangible material, such as a monolithic structure formed of metal or a brittle, fireproof ceramic material such as aluminum oxide, silicon dioxide, magnesium oxide, zirconia, cordierite, silicon carbide and the like. These materials provide a skeleton type of structure with a plurality of tiny flow channels. These structures are very fragile however. In fact, these monolithic structures are so fragile that small shockloads or stresses are often sufficient to crack or crush them.
The support structure is contained within a metal housing, with a space or gap between the external surface of the support structure and the internal surface of the housing. In order to protect the support structure from thermal and mechanical shock and other stresses noted above, as well as to provide thermal insulation, it is known to position at least one sheet of mounting material within the gap between the support structure and the housing. For example, U.S. Pat. Nos. 4,863,700, 4,999,168, and 5,032,441, each of which is incorporated herein by reference, disclose catalytic converter devices having a mounting material disposed within the gap between the housing and the support structure contained in the devices to protect the fragile support structure and otherwise hold it in place within the housing.
However, the conventional mounting materials used in these catalytic converter devices, while suitable for most current automotive catalytic converters, exhibit difficulties when the catalytic converter operating temperature is either very low (20.degree.-300.degree. C.) or very high (750.degree.-1200.degree. C. and above). For instance, when these conventional mounting materials have been used in catalytic converters for vehicles having a higher gross weight than normal gasoline powered passenger automobiles, they have exhibited failure. Because of their high gross vehicle weight, the engines of such vehicles operate at a much higher percentage of their maximum output for a much greater percentage of their operating time, than do the engines in passenger automobiles. These operating conditions in heavier vehicles result in maximum catalytic converter temperatures of much greater than 850.degree. C. In fact, converter monolith temperatures of 1050.degree. C. are not uncommon, and temperatures in excess of 1200.degree. C. may be encountered.
Heretofore, a typical passenger automobile catalytic converter, such as the one shown in U.S. Pat. No. 5,032,441, utilized a ceramic monolith which was supported by an intumescent mounting material having a nominal thickness of about 4.95 mm to about 9.9 mm and a nominal density of about 0.63 g/cm.sup.3. This material was compressed during installation of the ceramic monolith into its metallic housing to a nominal thickness of about 3.1 mm to about 6.2 mm and a nominal density of about 1 g/cm.sup.3. The conventional intumescent mounting material contains vermiculite which expands at about 300.degree. C. and degrades at temperatures greater than 750.degree. C. Thus, upon the initial heating of the catalytic converter assembly, particularly the initial cycles, conventional intumescent mounting materials experience a tremendous expansion pressure which is capable of crushing the catalyst support structure and causing component failure.
Accordingly, a need exists for maintaining a constant pressure on the metallic housing and the catalyst support structure under all conditions. In other words, a mounting mat which does not expand substantially upon initial heating of the catalytic converter assembly is seen as highly desirable.
While conventional intumescent mounting material meets the needs of most current automotive catalytic converters, it does not meet the needs of several near future requirements as well as some current diesel and heavy duty truck requirements. These requirements are focused upon the maintenance of near constant residual mounting pressure in temperature regimes below 300.degree. C. and above 750.degree. C. Conventional intumescent mounting material cannot provide such a constant pressure at these extreme temperature regimes.
Examples of severe condition applications in which these properties are important include the following: 1) Close-coupled converters which are mounted closer to the engine for better conversion efficiency via higher gas temperatures (about 750.degree. C.); 2) Diesel convertors and diesel particulate traps which operate at low temperatures and which are commonly pre-heated at 500.degree. C. to pre-expand the intumescent mat prior to installation in the vehicle. This "pre-heating" would be unnecessary with the mounting mat of the present invention. 3) Heavy-duty truck converters and motorcycle converters which run at temperatures which greatly exceed 750.degree. C. 4) Thin wall monoliths which will assist in meeting future EPA requirements via reaching operating temperature quicker due to their lighter mass. These monoliths are weak and will be crushed by the dramatic pressure increase of intumescent mounting mats.
Intumescent mounting mats would fail in the above cited severe condition application examples due to lack of expansion at low temperatures, to high pressure excursions between 300.degree.-750.degree. C., and to loss of pressure above 750.degree. C. With lack of expansion or loss of pressure the fragile monolith would be released, rattle about within the can, and self-destruct due to mechanical shock. With high pressure excursions, low strength monoliths would be crushed.
Alternative mounting mats, such as those having intumescent sheet material stitch-bonded thereto as described in U.S. Pat. No. 4,929,429, have also been investigated for severe condition applications. However, these mats have been found to be difficult and cumbersome to handle and to fabricate into catalytic converter assemblies. The mounting materials proposed to accommodate the severe condition applications are themselves fragile, and require expensive preprocessing such as stitchbinding prior to installation. Moreover, the mounting material used may require combination with other mounting materials, such as intumescent sheets and backing layers, in order to provide sufficient strength for handleability. These mounting materials are generally very thick and lack structural integrity, even being handled in a bag to prevent crumbling. Thus they are difficult to cut to size for installation, and further must be compressed substantially to fit enough material needed for supportive mounting within the gap between the catalyst support structure and the housing. Consequently, "flashing" commonly occurs, with excess material being squeezed out of the housing.
Another alternative mounting mat is shown in U.S. Pat. No. 3,771,967. No stitchbinding is employed in for the mounting mat of this patent. Instead, this patent discloses that the ceramic fiber layer or ring which comprises the mounting mat may be impregnated with a binder and a rigidizer to adhere the fibrous mounting material to the metal housing or shell. This mounting mat, hardened with binder and rigidizer, lacks the flexibility necessary to prevent the support structure from being crushed or otherwise damaged during use in the extreme conditions noted hereinabove. Other examples of alternate approaches besides those described in U.S. Pat. Nos. 4,929,429 and 3,771,967, are found in U.S. Pat. Nos. 4,693,338 and 5,028,397.
Diesel particulate traps similarly include one or more porous tubular or honeycomb-like structures (having channels closed at one end, however) which are mounted by a thermally resistant material within a housing. Particulate is collected from exhaust gases in the porous structure until regenerated by a high temperature burnout procedure, which thermally taxes the mounting material.